A liquid crystal display device is now being widely used attributed to its characteristics such as slim profile, light weight, and low electrical power consumption. The liquid crystal display device includes a pair of substrates and a liquid crystal layer interposed there between. Further, the liquid crystal device provides display by controlling an alignment direction of liquid crystal molecules contained in the liquid crystal layer by appropriately applying a voltage to electrodes arranged on liquid crystal layer side-surfaces of the substrates. The liquid crystal display device usually includes an alignment film for controlling the alignment direction of the liquid crystal molecules, and the alignment film is arranged on the liquid crystal layer side-surface of the substrate.
As a material for such an alignment film constituting the liquid crystal display device, resins such as polyamic acids, polyimides, polyamides, and polyesters are conventionally used. Among them, polyimides have been much used for liquid crystal display devices attributed to its excellent physical properties such as heat resistance, affinity with liquid crystals, and mechanical strength compared with other organic resins.
The alignment film is typically subjected to an alignment treatment, and thereby the film can provide liquid crystal molecules that are positioned on the alignment film surface with specific pretilt angles. A rubbing method, a photo-alignment method, and the like, are mentioned as a method for the alignment treatment. According to the rubbing method, the alignment film surface is provided with the alignment treatment by being rubbed with a cloth wound on a roller. According to the photo-alignment method, a photo-alignment film is used as a material for the alignment film, and the photo-alignment film is irradiated with (exposed to) light such as ultra violet light, and thereby the alignment film is provided with an alignment regulating force and/or an alignment regulating direction of the alignment film is changed.
However, according to a liquid crystal display device including a conventional alignment film, image sticking might be generated on a screen when an image is displayed for a prolonged period. So there is room for improvement in that the image sticking is suppressed even when an image is displayed for a prolonged period.
In order to solve such a problem, for example, Patent Document 1 discloses a liquid crystal aligning agent composition containing a tetrafunctional silicon composition like tetrachalcoxylan, a trifunctional composition like trialcoxylan, and a product of reaction with 0.8 to 3.0 mol water for a 1 mol functional group like an alkoxy group, and a glycol ether solvent, as a technology of providing a liquid crystal aligning agent capable of forming a liquid crystal alignment film which prevents display defects, has an excellent afterimage characteristic even after long-time driving, does not decrease the capability of aligning liquid crystal, and has a small decrease in voltage holding rate against light and heat.
In addition, for example, Patent Document 2 discloses a liquid crystal alignment material including a polyamic acid having a structure derived from a monoamine compound or its imidized polymer, as a technology of providing a liquid crystal alignment material which can develop good coating film formability and liquid crystal alignment characteristics and can form a liquid crystal alignment layer capable of deleting afterimages in a short time after the stop of the application of a voltage in liquid crystal display elements.
In addition, for example, Patent Document 3 discloses a vertical liquid crystal aligning agent including 100 weight parts of a polymer with an amic acid repeating unit and/or an imide repeating unit and at least 5 weight parts of a compound having at least two epoxy groups in a molecule, as a technology of providing a liquid crystal aligning agent which provides a vertical liquid crystal alignment film excellent in image sticking characteristics and reliability even when the film is used with a reflection electrode.
For example, Non-patent Document 1, which is a document on an photo-alignment film, discloses that the smaller an electrical resistivity of a photo-alignment film is, the shorter an image sticking time is.
Further, for example, Non-patent Document 2, which is a document on development of a material for alignment films, discloses that a reduction in residual DC in a liquid crystal cell with a vertical electrical field leads to suppression of the image sticking.
In an AC driving liquid crystal display device, a residual DC is generated generally due to a difference in an offset voltage between electrodes formed on substrates facing each other.
In addition, for example, Patent Document 4 discloses polyimides containing a side chain group having a structure that can be derived from 3-arylacrylic acid, as photoreactive polymers that can produce a stable and high-resolution alignment pattern that show a defined pretilt angle and at the same time has a sufficient high resistance value (holding ratio) in a liquid crystal medium adjacent to the pattern when the pattern is irradiated with polarized light.
For example, Patent Document 5 discloses polyimides, incorporating cinnamic acid group derivatives in such a way that the cinnamic acid groups are linked to the polyimide main chain via a carboxylic group by means of a flexible spacer, as photoreactive polymers that can produce a stable and high-resolution alignment pattern that show a very large tilt angle and at the same time has a sufficient high holding ratio in a liquid crystal medium adjacent to the pattern when the pattern is irradiated with polarized light.